Flat-panel displays are widely used in conjunction with computing devices, in portable devices, and for entertainment devices such as televisions. Such displays typically employ a plurality of pixels distributed over a display substrate to display images, graphics, or text. For example, liquid crystal displays (LCDs) employ liquid crystals to block or transmit light from a backlight behind the liquid crystals and organic light-emitting diode (OLED) displays rely on passing current through a layer of organic material that glows in response to the current. In recent years, low-resolution, high-brightness outdoor displays using inorganic light-emitting diodes (LEDs) have become popular, especially for advertising and in sporting venues.
Color pixels are provided in LCDs by color filters used to individually filter the light passing through each light-emitting element of the array of liquid crystals. All of the liquid crystals can be identical and enabled with a common power supply connection. White OLED display also use color filters and all of the OLED pixels are similarly identical and enabled with a common power supply. In contrast, color pixels are provided in RGB OLEDs by providing different organic materials that each emit different colors of light. These different organic materials can also be enabled with a common power supply.
In contrast, inorganic LEDs that emit different colors of light are often constructed in different materials, have different threshold voltages and current response, and require different power supplies. These different power supplies are provided externally and then distributed over the substrate or structure on or in which the array of inorganic pixels is located. Thus, for a three-color inorganic LED display, three different external power supplies capable of supporting the pixels associated with each color are needed together with sets of power lines that are routed and connected over the display area. Such connections can reduce emission area (aperture ratio), increase the cost of materials, and increase the number of interconnections, leading to reduced yields. Furthermore, batches of inorganic LEDs, even when made of the same materials in the same processes, tend to have a variable color output, a variable turn-on voltage, a variable resistance, and a variable current-response curve. Thus, when connected to a common power supply, the different inorganic LEDs will have different efficiencies and performance and the common-power circuits providing electricity to the different inorganic LEDs will have variable losses.
Integrated circuits of the prior art sometimes provide an on-chip power-conversion circuit to provide an additional power supply having a different voltage than the other circuitry of the integrated circuit. An example of one such power-conversion circuit is a charge pump, illustrated in FIG. 16. As shown in FIG. 16, an example prior-art charge-pump circuit includes two input voltage lines (e.g. power and ground) connected across an input capacitor CIN. A switch (S1) is connected in each of the input voltage lines and another capacitor, conventionally called the flying capacitor CFLY, is connected across the input lines after the switches S1. Another set of switches (S2) is connected in each of the input voltage lines after the first switches S1 and the flying capacitor and are connected to the output voltage lines and the terminals of an output capacitor COUT. Switches S1 and S2 are alternately operated, for example with a clock signal and an inverter, as shown. In operation over time the charge-pump circuit tends to provide the same voltage across the input lines and the output lines, as charge is pumped from the first capacitor to the flying capacitor to the output capacitor. By providing one or more of the charge-pump circuits with common input voltages and different connections between the output and input lines a variety of output voltages are achieved. For example, by connecting the V2OUT line to the V1IN line, the voltage across the V1OUT line and the V2IN line is twice that of the voltage across the V1IN and V2IN lines. The lower illustration of FIG. 16 is a representation of any of a variety of charge-pump circuits such as that of the upper illustration. Various charge-pump circuits connected in various ways provide a wide variety of different power and current sources.
Because of the variability in micro-LED (μLED) materials and manufacturing processes, different μLEDs, even when made in similar materials, have different performances and losses in the circuits providing power to the different μLEDs. μLEDs made in different materials have even greater inefficiencies when provided with a common power source. Furthermore, these issues are exacerbated in μLEDs since the variability of materials in a source semiconductor wafer is much greater on a smaller scale than on a larger scale.
There is a need, therefore, for improvements in power circuits for arrays of electronic elements such as inorganic μLEDs including different materials.